In a copending, concurrently filed patent application of W. Trimmer et al., Ser. No. 337,899 there is described a system premised on utilization of a stylettelike passive element for sensing sonic pulses in the body, and conveying those pulses, as sonic energy, out through the needle, where they are converted to electrical energy. Thus, a cylindrical needle probe carries a coaxial conduit rod, which terminates in the region of the needle tip, as desired either just outside, at, or within the needle tip. The stylette conduit is coupled to a transducer outside the body, such that ultrasound pressure waves which are conducted through the needle and out of the body are only then converted to electrical energy. Hence, the only portions of the apparatus which penetrate and/or contact body tissues are the electrically passive, essentially disposable and easily sterilizable needle and stylette portions. In preferred embodiments, the sonic conduit is fashioned of stainless steel, and terminates in a sonic matching layer. In some embodiments, the interior surface of the needle sheath is not physically isolated from the stylette, it being understood that during insertion, some mutual bending may occur, and some incidental contact may occur between the stylette and the needle. Other embodiments, however, seek to insure that minimal acoustic energy transfer will occur between the stylette and the interior of the needle sheath. Thus, as will be particularly noted from FIGS. 7A and 7B, and FIGS. 8A and 8B of the concurrently filed copending application of Trimmer et al., annular or helical ridges, or an overall helical stylette shape create periodic points of contact between the stylette and the interior needle surface. These points of contact are, hopefully, sufficiently small in area, or oblique in angle of contact whereby the sonic pressure wave moves directly up the stylette, and is not substantially transferred to the needle sheath interior. Similarly acoustical energy from the exterior of the needle is substantially transferred to the stylette.
It is an object of the present invention to provide acoustic isolation between the sonic energy carrying stylette, and the surrounding needle in such systems. It is a more particular object of the principles of the present invention to provide the sorts of acoustical interface which avoid even point contact interface between the sonic conduit (i.e. stylette) and the surrounding hollow needle.